Inside the police station, Joseph Boston told investigators that, yes, he had lured two boys into his motel room and molested them – but there was much more to his story than that, according to police in Riverside, California.

Riverside police said that on Saturday the 18-year-old went on to detail how he had molested “upwards of 50 children” in cities across southern California for nearly a decade. He’d been molesting children since he was 10, police said.

The open confession rattled investigators, who spent the weekend retracing Boston’s tracks over nearly half his life, looking for a trail of young victims, police said.

“This is going to affect not only the victims for a long time but also our detectives and officers involved in this – to hear someone just be very open about what they’ve done, and they’re only 18 themselves,” Officer Ryan Railsback, of the Riverside police, told Los Angeles ABC-affiliate KABC.

Boston’s mother contacted police around 3am on Saturday, saying she wanted to turn her son in for suspected molestation, police said in a news release. According to police, Boston had called his mother and said he had molested two children a short time earlier.

He had been staying at the Simply Home Inn & Suites, a motel that caters to travellers in Riverside. There, he had “befriended” an eight-year-old and a four-year-old, brothers who were staying at the hotel with their parents, the police statement said. The parents allowed the children to go into Boston’s room on Friday night, where the molestation happened, police said.

Police brought him in for questioning, where they said he made the bigger revelations. Meanwhile, the boys who police suspected were Boston’s victims Friday were taken into custody by the County of Riverside Child Protective Services.

It was unclear why the children’s parents might have allowed the boys to go with Boston.

Boston is charged with oral copulation on a child under the age of 10 and has been booked into jail with bail set at $1 million. He’s scheduled to make his first court appearance on Wednesday, according to the Associated Press. It wasn’t immediately clear if he had hired an attorney.

According to investigators, Boston has lived in Riverside since early November. Before that, he lived in Lakewood and Buena Park in southern California.

A Facebook page alleged to be Boston’s was taken down but not before several news organisations had taken images of posts that seem chilling in hindsight.

One shows a person alleged to be Boston lying on a floor or a bed with a young girl by his side. He’s looking at the camera, and she appears to be playing with a toy. There’s a container of food open nearby. The emoji-filled caption on the post reads: “I like Making New friends.”

Another post shows a photo of a sleeping boy wearing a Batman T-shirt. “What a cute kid,” the caption reads. “And they are way easier to control when they are sleeping and not in a bad way!!”